Optoelectronic integrated circuits (ICs) include both electronic and optical elements within a single chip. Typical electronic elements include field effect transistors, capacitors, and, resistors; typical optical elements include waveguides, optical filters, modulators, and photodetectors. Within a given optoelectronic IC, some of the electronic elements may be dedicated to handling tasks such as data storage and signal processing. Other electronic elements may be dedicated to controlling or modulating the optical elements. Including both types of elements on a single chip provides several advantages, which include reduced layout area, cost, and operational overhead. In addition, such integration yields hybrid devices, such as an opto-isolator.
The integration of optical and electronic elements has been greatly facilitated by the maturation of today's semiconductor processing techniques. For instance, conventional processing techniques may be adapted to create silicon-based prisms, waveguides, and other optical devices.
One device, however, that has been difficult to integrate is a silicon based laser or light source. As a result, most optoelectronic ICs are adapted to receive an externally applied light beam from a laser or an optical fiber. Unfortunately, introducing a light beam to an IC can often be difficult. For example, in order for an optoelectronic IC to accommodate a light beam, the spot size and the numerical aperture (NA) of the beam may need to be appropriately matched to optical elements within an IC.
To overcome these difficulties, previous methods use a silicon-based prism to couple a light beam into and out of a waveguide. To couple light into the waveguide, the prism receives an externally applied light beam and refracts it to a coupling region. The coupling region then provides the refracted light beam to the waveguide. A reverse scenario moves a light beam away from the waveguide, into a coupling region, which transfers the light beam into the prism. The prism then refracts the light beam out of the prism. To provide an evanescent coupling, the prism should be located within close proximity of the waveguide and should have an index of refraction that is greater than or equal to that of the waveguide. Further, the separation distance between the prism and waveguide should be small (on the order of 1000's of Angstroms) and the index of the medium that separates the two must be smaller than the indexes of both the prism and the waveguide. In many scenarios, the index of the prism (n1) is larger than the index of the separation medium (n2) and larger than or equal to the index of the waveguide (n3).
In general, optoelectronic ICs are fabricated in SOI based substrates. Advantageously, SOI substrates provide a thin device layer located on top of a buried oxide (BOX) layer. The device layer is used for a waveguide and the BOX layer serves as a cladding layer of the waveguide. FIG. 1 shows an SOI substrate 10 that includes a device layer 12, a BOX layer 14, and a bulk layer 16. The device layer 12 includes a waveguide 18. Above the waveguide 18 are a coupling region 20 and a prism 22 for optically coupling a light beam into the waveguide 18. The coupling region 20 is an oxide layer that is either grown or deposited between the waveguide 18 and the prism 22 The prism 22 is a silicon based prism that is bonded to the coupling region 20. The BOX layer 16 and a thick oxide layer 24 serve as cladding layers of the waveguide 18, and both layers 16, 24 have a thickness that is on the order of 1 um for sufficiently confining a light beam to the waveguide 18.
Additionally, FIG. 1 shows a silicon-oxide-silicon modulator 28 located above the waveguide 18 (where a portion of the waveguide 18 silicon serves as a lower plate in the modulator 28) and a field effect transistor (FET) 30 adjacent to the waveguide 18. An oxide region 32, interposed between the FET 30 and the waveguide 18, provides electrical isolation. When the modulator 28 receives a voltage bias, it may adapt the optical properties of a light beam within the waveguide 18. The FET 30 may be included in a variety of modulator bias controls, signal processing, switching, or data storage related circuits, for instance.
To effectively couple a light beam into the waveguide 18, the prism 22 includes a tapered surface 34, which allows a variety of incident angles to be achieved. The prism 22 may also include an anti-reflective coating that reduces Fresnel losses. FIG. 1 shows a light beam 36 incident on the tapered surface 34. Typically light beams used in optoelectronic ICs are in the infrared range of about 1-5 μm, which is transparent to silicon. The prism 22 refracts the beam 36 and the coupling region 20 then assists in coupling the beam 36 to the waveguide 18. The waveguide 18 may then provide the beam 36 to other portions of an IC, or another optical coupling structure, which may draw the beam 36 out of the waveguide 18. For instance, FIG. 1 shows a prism 38 and an coupling region 40, which together may draw the beam 36 out of the waveguide 18.
One problem with current optoelectronic coupling structures is that bonding the prism to the coupling region is difficult. Typically, prisms are fabricated separately from a substrate and then later aligned to a coupling region and bonded. The alignment and bonding can be a labor intensive, time consuming task, which reduces throughput and makes larger scale production less feasible. In addition, interconnects, which electrically couple together various electronic elements, must also accommodate a prism. Often this leads to an increased number of restrictions in the layout rules of the contacts, vias, metal interconnects, and electrical and optical elements within a given optoelectronic IC.